100% Turkish emery sand. This is the same emery used in emery boards and those little strawberries that come on some 'tomato' pin cushions. Emery sand has long been known to sharpen needles but it also cleans them. The square bottle has a slender neck to pour easily into your pin cushion.
Because this is so fine, I do recommend sewing it within its own casing so it will have two layers altogether.